No, if the fan changed direction, it would first off reverse the airflow, plus the efficiency of the fan would be worse, since the design of the fins and sometimnes frame of the fan is designed to improve airflow in one direction with the best possible efficiency.
Fans are usually optimized to move air in one direction, but not the opposite. If a fan had to work equally well in both directions, the fin design would be very different
I'd think your eyes are maybe decieving you, the LED lights in a PC can sometimes make the rotation of a fan look strange if the light shines on the fan's fins when they rotate, due to the way LEDs emit light
There are a couple of fans I have seen that will spin opposite at initial startup to do a bit of dust cleaning, but this isn't something common. I think the last one I recall seeing this on was an Enermax heatpipe cooler and it was touted as a feature.